Considering the possibilities of using plant oils as raw materials for the synthesis or modification of epoxy resins, an interesting solution would be just to give the plant oils proper functionality so that they can also act as natural hardeners. The use of modified vegetable oils in this role is favored by their very good miscibility with the epoxidized vegetable oils and good miscibility with the epoxy resins based on bisphenol A (despite the differences in the polarity of both groups of materials). For example, hydrolyzed castor oil can serve as a source of dehydrated fatty acids from which reactive polyamides are obtained by reaction with acrylic acid [141] (Figure 1.54) and then with various polyamines (diethylenetriamine, triethylenetetramine, and tetraethylenepentamine) [17].
The obtained polyamides (with amine values from 310 to 389 mg KOH/g) can be used to cross‐link the epoxy resins based on bisphenol A giving the materials with good coating properties.
Figure 1.54 Synthesis of C21 cycloaliphatic dicarboxylic acids and reactive polyamides from them.
Figure 1.55 Scheme of soybean oil functionalization with thioglycolic acid.
Vegetable oil‐based curing agents can be obtained through direct oil modification. The novel bio‐based polyacid hardener is synthesized by the thiol‐ene coupling of soybean oil with thioglycolic acid (Figure 1.55) [142].
The synthesized soybean oil‐based polyacid exhibit a functionality of 3.3 acid functions per triglyceride molecule. This polyacid curing agent is characterized by the high reactivity toward epoxy groups. The low molecular weight bisphenol A‐based epoxy resin cross‐linked with the bio‐polyacid exhibits interesting properties for coating and binders (the Shore A hardness of 52 and E′ = 0.59 MPa). A commercially available mercaptanized soybean oil (prepared by direct addition of H2S to soybean oil) reacted with allylamine (Figure 1.56) or its salts can also be potentially used as a cross‐linking agent for epoxy resins [143].
Modified lignin derivatives are also applied as curing agents for the epoxy resins. Commercially, various curing agents for epoxy resins are available. The commonly used petroleum‐based curing agents include amines, amides, hydroxyls, acid anhydrides, phenols, and polyphenols. However, recently, a great effort has been put on obtaining new bio‐based hardeners for epoxy systems. Generally, lignin‐based curing agents are prepared by two different methods: (i) the reaction of lignin with ozone in the presence of NaOH to give lignin with unsaturated carboxyl groups (Figure 1.57) or (ii) throughout the reaction of modified lignin (partially depolymerized lignin or polyol solutions of alcoholysis lignin) with anhydrides or trimellitic anhydride chloride [144].
Figure 1.56 Synthesis of a polyamine cross‐linking agent via thiol‐ene reaction of mercaptanized soybean oil with allylamine.
Figure 1.57 Synthesis of carboxylic acid from lignin.
Another interesting approach is using aminated lignin (black powder, amine value: 180–200 mg KOH/g) as a cross‐linker of bisphenol A‐based epoxy resin (epoxy value: 0.48–0.54 mol/100 g) [145]. The obtained aminated lignin contains a large number of primary and secondary amine groups, which successfully cure the epoxy network (Figure 1.58).
The application of aminated lignin has the positive effect at the initial degradation stage of the epoxy resin. Because lignin itself has a good thermal–mechanical performance, samples prepared with its higher content presented accordingly improved properties (Table 1.6).
TGA and DMA tests reveal improved thermal–mechanical properties of the bio‐based epoxy resin. In comparison to the epoxy resin cross‐linked with the commercial curing agent based on modified isophorone diamine (W93 curing agent: amine value: 550–600 mg KOH/g), the recorded value of T10, for the material cured with the hardener containing 20 wt% of aminated lignin, increased nearly by 50 °C, while the highest value of T10 was achieved for the epoxy system cured by 100% aminated lignin (T10 = 266 °C for 100% of petrochemical‐based hardener W93, T10 = 315 °C and T10 = 332 °C for aminated lignin hardener, 80%W93 + 20%AL and 100%AL, respectively). Additionally, the mass loss before 300 °C of the epoxy resin cured by W93 is four times higher than the one recorded for the aminated lignin. Moreover, the obtained materials are characterized by improved values of the glass transition temperature and thermal deformation temperature. Tg and Td of epoxy resin sample cured with the hardener containing 50 wt% of lignin increased by 14 °C compared with the one without lignin.
Figure 1.58 The curing of epoxy resin using aminated lignin as a curing agent.
Table 1.6 TGA, Tg, and Td values of epoxy resin samples cured with different contents of lignin in the curing agent [145].
| Epoxy resin cured by different hardeners | Total mass loss before 300 °C (%) | T10 (°C) | T50 (°C) | Tg (°C) | Td (°C) |
|---|---|---|---|---|---|
| 100%W93 | 11.1 | 266 | 362 | 79 | 70 |
| 80%W93 + 20%AL | 8.7 | 315 | 370 | 86 | 74 |